The present invention relates to a system for recording electrical waveforms and more particularly to such a system employing a charge transfer device.
The high speed recording of photon images involves both mature and developing camera technologies which span the electromagnetic spectrum from infrared to x-rays, with temporal resolutions of micro- to femto-seconds. Often, the cameras, like streak cameras, are very expensive, bulky or involve precision machinery, such as rotating mirrors.
For many applications, it would be advantageous to have small, solid state imaging devices manufactured and replicated by the methods of the semiconductor circuit industry. A solid state imager necessarily includes three elements: (1) an input comprised of an array of image sensors, to transduce the incident photon energy into charge carriers in the device; (2) a charge transfer technique, to move the charges from input to an output in an organized fashion; and (3) the output, which may involve storage, display, or further processing. The arrival of alternate technologies for any of these three elements can enable the development of a new class of imagers. In particular, alternate technologies for solid state charge transfer devices (CTDs) are now available.
The first CTD, originally conceived by N. Wiener (Cybernetics, John Wiley and Sons, Inc., New York, 1948, p.144), was the Bucket Brigade Device (BBD), in which charge is stored in a serial array of capacitors and transferred by proper activation of switches placed between them. Several versions of the BBD were later developed utilizing mechanical switches, vacuum tubes, or bipolar transistors as the switching elements. The fully integrated circuit version came about in 1970, when the switches were replaced by MOS transistors. See F. L. J. Sangster, "Integrated Bucket Brigade Delay Line Using MOS Tetrodes," Phillips Tech. Review 31;266.
It was in 1970 when W. S. Boyle and G. E. Smith, "Charge Coupled Semiconductor Devices," Bell Syst. Tech. Jour. 49, 1970, pp.587-593, conceived and demonstrated the first Charge Coupled Device (CCD). In its original version the CCD consisted of closely spaced electrodes on an isolated surface of a semiconductor. With a proper sequence of pulses applied to these electrodes, packets of minority carriers were transferred along the surface of the semiconductor. The impact of the CCD was immense and swift. Almost immediately dozens of schemes and variations of the device were under study and many potential applications were in the making.
Charge Coupled Devices play an important role as self scanned light sensors. See C. H. Sequin, "Image Sensors Using Surface Charge Coupled Devices," in Solid State Imaging, edited by P. G. Gespers, F. Van der Wiele and M. H. White, Noordhoff-Leyden, 1976, pp. 305-329. Since silicon is a photosensitive material, charge packets may be created by a light pattern directed onto the surface of the semiconductor. These packets are transported to the output port of the CCD and read out sequentially. The CCD can also perform analog signal processing functions such as high density storage (e.g., of an image), analog delay and transversal filtering.
Initial interest in the CCD was heightened by the striking simplicity of its structure and the range of potential applications. However, as the CCD technology progressed the device performance improved on the one hand while its complexity increased on the other hand. In practical devices the electrodes are driven by multiphase clocks (up to four phases) requiring complex peripheral circuitry. Multilevel electrodes and buried channels are required to improve the efficiency and speed. For imagers, the clock busses occupy a large fraction of the available area. These drawbacks limit the speed, density and simplicity of the device.
In the realm of nonrepetitive electrical waveforms, the fastest recorders are oscilloscopes, which operate on the principle of deflecting an electron beam. A common way to achieve fast response is to compromise the sensitivity of the oscilloscope. Compensation for the loss in sensitivity can sometimes be made by employing amplifiction in the electronics, which is difficult and expensive for fast pulses.
Digital recorders of nonrepetitive waveforms are even more expensive and generally have less bandwidth than state-of-the-art oscilloscopes.
For repetitive signals, sampling scopes offer better temporal resolution than oscilloscopes. One reason sampling scopes are used, is to obtain temporal resolution of the waveform, even if that requires the added inconvenience of sampling many repetitions of the waveform. If superior temporal resolution were available, many applications of the expensive sampling scopes would be obviated.
U.S. Pat. No. 4,389,590; "System for recording waveforms using spatial dispersion", R. R. Whitlock (1983) discloses devices that allow for the recordation of electrical waveforms, be it from an electrical source or a photon source, and allows for bandwidths of greater than 1 GHZ. However, one major limitation of this device is that the transfer time across a gate is no faster than one period of the wave. Another major limitation is the requirement for two waves.